**3.1 Gas parameters, gas quality figures (G 260, G 685)**

The gas used in the networks for the final customer has to fulfill quality and composition requirements. According to the standard defined by DVGW G 260 working sheet two main types of natural gas (gas families) are distinguished which stem from different sources and production locations:


Aside from the composition of the gas, for technical reasons the values of calorific value and Wobbe-Index are important characteristics. A typical range of these values is used in practice and will be permanently measured and surveyed:

Gas Quality Parameter Computation in Intermeshed Networks 139

The mixing of the flows and thus the resulting value is not perfect in the vicinity of the mixing point. This is due to the pipe dimensions which may be large or different and the flow characteristic: laminar or turbulent. The mixing process is better and faster if the flow is turbulent. If the flow is laminar mixing may take a long way and time as even a layering effect may occur. In order to speed up mixing in such cases static mixer pipes, which have

Streams of gas and fluids show a typical behaviour when the flow changes from laminar state to the turbulent state (see figure 4). The areas of the flow type are described by the Reynold number (Re). Beyond Re= 2300 the flow switches suddenly from laminar to turbulent. But he transition spreads over a range and is dependent on physical and technical parameters. In pipeline systems the main parameters are pipeline inner diameter and roughness value which determine the flow type inside. Generally spoken, smaller diameters and higher roughness values lead to turbulent flow (but also to a higher pressure loss which is not really wanted).

small obstacles inside to provoke little turbulence, will be built in the line.

(*Re* is defined as*: Re = v d / ʋ* , (*v* = velocity, *d* = pipe diameter, *ʋ* = dynamic viscosity))

**3.4 Gas conditioning, process optimization** 

Fig. 4. Laminar and turbulent flow in Reynold number vs. pipe roughness number

Gas conditioning is required in biogas plants in order to provide gas of near or equal quality of natural gas when it is fed into the network. Many consumers, especially industrial

**3.3 Laminar vs. turbulent flow** 

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Table 1. Essential gas parameters

The calorific value is generally used for the billing, as the final consumer/customer must receive his bill with the energy value included, meaning in the unit of kWh in a period (i.e. a year, a month). The energy value yields from multiplication of accumulated flow and calorific value, e.g. 3000 m3/a \* 10.1 kWh/m3 equal to 30300 kWh/a.

#### **3.2 Basic equations for mixing**

Gas mixing occurs when streams of different gas qualities are united to a single stream. In pipeline systems this means that different gas sources meet in a T-type or Y-type of pipeline. A simple example of two streams of volume (V) with two calorific values (H) is given below (see figure 3). The resultant value depends on the product of volumes or flow (Q) and the amount of each calorific value according to:

$$\mathbf{H}\_{\mathbf{1},2} = \frac{\mathbf{V}\_{\mathbf{1}} \ast \mathbf{H}\_{\mathbf{1}} + \mathbf{V}\_{\mathbf{2}} \ast \mathbf{H}\_{\mathbf{2}}}{\mathbf{V}\_{\mathbf{1}} + \mathbf{V}\_{\mathbf{2}}}$$

$$\mathbf{H}\_{\mathbf{s}} = \frac{\sum\_{\mathbf{i}} (\mathbf{V}\_{\mathbf{i}} \ast \mathbf{H}\_{\mathbf{i}})}{\sum\_{\mathbf{i}} \mathbf{V}\_{\mathbf{i}}} = \frac{\sum\_{\mathbf{i}} (\mathbf{Q}\_{\mathbf{i}} \ast \mathbf{H}\_{\mathbf{i}})}{\sum\_{\mathbf{i}} \mathbf{Q}\_{\mathbf{i}}}$$

H1,2 , Hs = resulting calorific value V1 , V2 = volume of stream #1, #2 H1 , H2 = calorific value of stream #1, #2 Qi = flow of stream i ( = dV/dt)

Fig. 3. Mixing and dynamic tracking of gas parameters at point K3

The mixing of the flows and thus the resulting value is not perfect in the vicinity of the mixing point. This is due to the pipe dimensions which may be large or different and the flow characteristic: laminar or turbulent. The mixing process is better and faster if the flow is turbulent. If the flow is laminar mixing may take a long way and time as even a layering effect may occur. In order to speed up mixing in such cases static mixer pipes, which have small obstacles inside to provoke little turbulence, will be built in the line.
